SUMMARY There are fundamental gaps in our understanding of how neoplastic cells tolerate the oncogenic stress and intrinsic DNA damage that arises during tumorigenesis, while simultaneously accumulating mutations that fuel cancer. Unfortunately, the DNA damage tolerance and mutability acquired during carcinogenesis also allow cancer cells to resist therapy. Filling the current gaps in our knowledge of DNA damage tolerance will allow us to harness intrinsic and therapy-induced DNA damage to kill cancer cells. Our long-term goal is to solve the problem of how cancer cells endure oncogenic stress and DNA damage. We recently discovered that cancer cells commonly depend on aberrant activation of two major genome maintenance pathways (Trans-Lesion Synthesis or TLS, and Homologous Recombination or HR) for DNA damage tolerance. This reliance on 'pathologically-activated' DNA repair is a new molecular vulnerability of cancer cells and provides opportunities for highly selective targeted therapies. The objective here is to define signaling mechanisms by which cancer cells activate TLS and HR. Our central hypothesis is that pathological DNA repair activity sustains cancer cell growth and confers resistance to therapy. The rationale is that defining the mechanisms of pathologically- activated DNA repair will reveal therapeutic strategies that target specific vulnerabilities of cancer cells. We will test our central hypothesis and attain our objectives using the following Specific Aims (SAs): SA1 Elucidate structural basis for RAD18 activation by MAGE-A4. SA2 Define contribution of pathologically- activated Trans-Lesion Synthesis (TLS) to oncogenic stress tolerance and carcinogenesis in vivo. SA3 Define novel mechanism by which Homologous Recombination (HR) is pathologically activated via HORMAD1 in cancer. SA1 will use biophysical methods and new peptide probes to elucidate the mechanism by which MAGE-A4 interacts with RAD18. In SA2 mutant mice lacking Rad18 (the apical mediator of TLS) or mice overexpressing MAGE-A4 (a cancer-specific activator of TLS) will be used to determine how TLS impacts tumorigenesis and the genomic landscape of oncogene-driven cancers in vivo. For SA3 we will use cell culture models to determine how the cancer/testes antigen HORMAD1 (which is aberrantly over-expressed in cancer cells) signals activation of DSB repair, oncogenic stress tolerance and radioresistance. We propose innovative new solutions to the important problems of how oncogenic stress tolerance and mutability arise, drive carcinogenesis, and lead to therapy resistance. The proposed work is significant because we will provide new paradigms for genome maintenance that are relevant to environmental exposures, mutagenesis, tumorigenesis and cancer therapy in humans. This work will lead to novel therapeutic strategies that target DNA damage tolerance specifically in cancer cells, thereby enhancing the efficacy and selectivity of existing anti-cancer agents.